TM 5-814-8
Table 8-5. Advantages and disadvantages of
Table 8-4. Thickening characteristics of various
using sludge drying beds
sludge types (percent solids) (167)
Centrification
Advantages
Disadvantages
Gravity
Air
(solid bowl
f. Higher dry cake solids con- f. May be more visible to
type)
Flotation
Thickener
Sludge Type
the general public.
tents than fully mechanical
Raw Primary
28-35
5-7
8-12
methods.
12-15
2-3
3-6
Activated Sludge
4-7
15-20
3-7
Trickling Filter
(b) Vacuum filters. Vacuum filters con-
4-6
6-8
18-24
Primary & WAS
sume more energy per unit of sludge dewatered
(4) Dewatering. Dewatering is the removal of
than drying beds and are labor intensive. Perfor-
water from wastewater treatment plant solids to
mance data for vacuum filters is presented in
achieve a volume reduction greater than that
table 8-6.
achieved by thickening. Dewatering is done pri-
Table 8-6. Typical sludge concentrations produced
marily to decrease the capital and operating costs
by vacuum filtration
of the subsequent direct sludge disposal or con-
Rate
Solids
Cake
version and disposal process. Dewatering sludge
(lb/hr/cu ft)
(percent)
Sludge Type
from a 5 to a 20 percent solids concentration
25-30
5-10
Raw Primary
reduces volume by three-fourths and results in a
20-25
3-6
Primary & Activated Sludge
non-fluid material. Dewatering is only one compo-
2-5
12-18
Activated Sludge
nent of the wastewater solids treatment process
4-6
28-32
Digested Primary
20-24
3-5
and must be integrated into the overall waste-
Digested Primary &
Activated Sludge
water treatment system so that performance of
both the liquid and solids treatment schemes is
(c) Belt presses. Belt press performance is
optimized and total costs are minimized. The
highly dependent upon chemical addition, pres-
dewatering processes discussed in chapter 7 in-
sure, cloth type, etc. and it is difficult to general-
clude: drying beds, vacuum filters, belt presses
ize their operating efficiency. Table 8-7 has been
and plate presses.
prepared as a summary of the reported minimum
(a) Drying beds. Drying beds are the most
and maximum cake solids for various types of
common type of dewatering equipment in use at
sludges.
military installations today. Drying beds are used
throughout the United States in small and large
Table 8-7. Typical dewatering performance of
treatment systems; however, their use has de-
belt filter presses
clined over recent years. Their most common use
Polymer
is in drying of domestic wastewater sludge but
Cake Solids Feed Solids lb/ton of
some industries also use this method. Table 8-5
percent
dry solids
percent
Sludge Type
lists the advantages and disadvantages of sludge
2-9
28-24
3-10
Raw Primary
16-32
1-3
2-4
dry beds.
Activated Sludge
Primary & Activated
Table 8-5. Advantages and disadvantages of
4-12
12-28
0.5-1.5
Sludge
using sludge drying beds
Anaerobically Digested
Disadvantages
Advantages
4-8
3-4
18-22
Activated Sludge
Metal Hydroxide
Requires more land than
a. When land is readily avail- a.
3-5
2-6
35-50
Sludge
fully mechanical methods.
able, this is normally the
lowest capital cost.
Removal usually labor in-
b. Small amount of operator b.
tensive.
attention and skill is re-
filters have been proven to yield the highest cake
quired.
solids concentration of all the dewatering meth-
Lack of a rational engi-
c. Low energy consumption. c.
ods discussed. A disadvantage of the units is a
neering design approach
high capital and labor cost and its requirement
allowing sound engineer-
ing economic analysis.
that it be operated in a batch mode. Table 8-8
d. Less sensitive to sludge d. Must be designed with
provides ranges of performance of filter presses
careful concern for cli-
variability.
on various sludges. Additionally, cycle times may
matic effects.
be as long as 6 to 8 hours per batch before
e. Low to no chemical con- e. Requires a stabilized
optimum cake solids is achieved.
sludge.
sumption.
8-13
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